Use <%= javascript_include_tag :defaults %> in the HEAD
section of your page (recommended): This function will return references to
the JavaScript files created by the rails command in your
public/javascripts directory. Using it is recommended as the
browser can then cache the libraries instead of fetching all the functions
anew on every request.

Use <%= javascript_include_tag 'prototype' %>: As above, but
will only include the Prototype core library, which means you are able to
use all basic AJAX functionality. For the Scriptaculous-based JavaScript
helpers, like visual effects, autocompletion, drag and drop and so on, you
should use the method described above.

Use <%= define_javascript_functions
%>: this will copy all the JavaScript support functions within a
single script block. Not recommended.